Hypothalamo-hypophysial relations in amphibian larvae

Abstract

The results of studies on the differentiation of the preoptic nucleus and of experiments regarding the effects of propylthiouracil and extirpation of the preoptic area demonstrate that in Xenopus laevis tadpoles a thyrotropin-releasing factor (TRF) is formed in peptidergic cells in the dorsal part of the preoptic nucleus. The differentiation of this nucleus depends on thyroid hormones, but at the same time these hormones inhibit the activity of the TRF cells.

Young larvae of X. laevis acquire the ability to adapt themselves to a white background simultaneously with the appearance of aminergic neurosecretory centers in the caudal hypothalamus. During prometamorphosis these centers comprise a paraventricular organ (PVO), a nucleus infundibularis, fiber tracts to the telencephalon and to the pars intermedia of the adenohypophysis, and a nucleus in the caudalmost part of the dorsal tuber cinereum. The PVO consists of sensory nerve cells and cells secreting products into the cerebrospinal fluid; the infundibular nucleus contains numerous liquor-contacting neurons. The aminergic neurosecretory cells contain catecholamines such as dopamine. Depletion of these cells by reserpine leads to a dispersion of melanin granules in the skin melanophores. It is believed that among the functions of the aminergic neurosecretory centers is the production of a melanotropin-inhibiting factor. In the ventral tuber cinereum of X. laevis, neurosecretory cells are situated that are neither peptidergic nor aminergic. They are more active in adult animals than in tadpoles